Joined: Sat Aug 19 2006, 05:03PM
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flj wrote ... There is lots of good information in the replies above. I guess what I was trying to say and did not say very well is that I believe that the factory proportioning valve with a front disc rear drum setup gives the discs significantly more pressure than the drums. I will pick some number here. Lets say the discs get 1200 psi and the drums get 600 psi max pressure from the factory proportioning valve. If I go to rear discs and they need 1000 psi there is no way that they can get that if the factory proportioning valve is left in the sytstem and all that I can do with an adjustable proportioning valve added into the rear line is to drop the max psi to the rear even more.
The aftermarket adjustable prop valve would replace the factory prop valve in the circuit, not be plumbed inline with the factory prop valve. If you need more pressure to the back brakes, just adjust the dial on the valve to not drop the pressure as much.
Joined: Tue Oct 11 2005, 01:33AM
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Great work up Feets. !thumb
Two points ...
Disc pads virtually rest on the rotor when the brakes are released, so when the brake pedal is depressed it moves some fluid, but mostly builds pressure. As long as the slack in the system is removed before you run out of pedal, you got brakes.
Secondly, they work and they work well as long as the reservoirs are kept topped up to allow for increased fluid in the system as pads wear.
Joined: Wed Aug 11 2010, 10:15AM
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Mike66Chryslers wrote ...
flj wrote ... There is lots of good information in the replies above. I guess what I was trying to say and did not say very well is that I believe that the factory proportioning valve with a front disc rear drum setup gives the discs significantly more pressure than the drums. I will pick some number here. Lets say the discs get 1200 psi and the drums get 600 psi max pressure from the factory proportioning valve. If I go to rear discs and they need 1000 psi there is no way that they can get that if the factory proportioning valve is left in the sytstem and all that I can do with an adjustable proportioning valve added into the rear line is to drop the max psi to the rear even more.
The aftermarket adjustable prop valve would replace the factory prop valve in the circuit, not be plumbed inline with the factory prop valve. If you need more pressure to the back brakes, just adjust the dial on the valve to not drop the pressure as much.
I do understand what you mean but the factory proportioning valve has two lines in from the master cylinder (1 front and 1 rear) and three lines out (2 front and 1 rear). I cannot replace the factory proportioning valve with a simple valve with one line in and one line out unless I was going to have completely separate front and rear brake lines with a separate T installed in the front brake line. I do not think that is a good idea. I am thinking about using the aftermarket 4 wheel disc proportioning valve that I listed originally which has a 50:50 front to rear bias and then adding the adjustable valve into the rear brake line. That should get over the fact that the factory proportioning valves will not give me enough pressure into the rear line and will also allow me to adjust the rear brake pressure down if needed.
Joined: Sat Aug 19 2006, 05:03PM
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flj, Even though the front brake circuit goes through the factory 1973 prop valve, the prop valve only affects the rear brake circuit, just as the aftermarket one does. There are two reasons that the factory prop valve has the front brake lines going into it:
1. The government mandated a warning light on the dashboard to indicate if one of the brake circuits failed. There is a slider block inside the unit that will slide to one side and close an electrical contact if one circuit loses pressure. You do not care about this. This slider block does not do anything magical like increase brake pressure to the circuit that has not failed. It only closes an electrical switch.
2. The factory prop valve replaced having a separate tee for the front brake lines as a production expedient. You can safely use a brass tee for the front brakes. No problem.
DO NOT plumb two prop valves in series. You are asking for trouble.
EDIT: I just noticed that you are converting a 67 Polara, which will already have dual-circuit brakes and a distribution block with the slider inside to activate an idiot light on the dash. (On a drum/drum car, this is a simple distribution block, not a prop valve.) Retain this block, connect the disc master cylinder in place of your existing drum MC, and plumb the adjustable prop valve into the line going to the rear brakes AFTER the distribution block. Done!
Next question: What brake booster are you going to use? Your original drum brake booster will not suffice.
<span class='smallblacktext'>[ Edited Thu May 12 2011, 05:33AM ]</span>
Joined: Sun Jul 18 2010, 11:11PM
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Fury440 wrote ... Disc pads virtually rest on the rotor when the brakes are released, so when the brake pedal is depressed it moves some fluid, but mostly builds pressure. As long as the slack in the system is removed before you run out of pedal, you got brakes.
Keep in mind that new brake systems have springs that keep the pads away from the rotors to reduce drag. It's not much of a gap but it is there.
I left all kinds of stuff out of that long winded ramble. The idea was to cover the basics in a way that people might be able to understand.
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Mike66Chryslers wrote ... flj, Even though the front brake circuit goes through the factory 1973 prop valve, the prop valve only affects the rear brake circuit, just as the aftermarket one does. There are two reasons that the factory prop valve has the front brake lines going into it:
1. The government mandated a warning light on the dashboard to indicate if one of the brake circuits failed. There is a slider block inside the unit that will slide to one side and close an electrical contact if one circuit loses pressure. You do not care about this. This slider block does not do anything magical like increase brake pressure to the circuit that has not failed. It only closes an electrical switch.
2. The factory prop valve replaced having a separate tee for the front brake lines as a production expedient. You can safely use a brass tee for the front brakes. No problem.
DO NOT plumb two prop valves in series. You are asking for trouble.
EDIT: I just noticed that you are converting a 67 Polara, which will already have dual-circuit brakes and a distribution block with the slider inside to activate an idiot light on the dash. (On a drum/drum car, this is a simple distribution block, not a prop valve.) Retain this block, connect the disc master cylinder in place of your existing drum MC, and plumb the adjustable prop valve into the line going to the rear brakes AFTER the distribution block. Done!
Next question: What brake booster are you going to use? Your original drum brake booster will not suffice.
The car is a 1967 Polara Convertible which had the Budd front discs. I will be replacing the Budd discs with the 1973 Kelsey Hayes system. My booster should be adequate for the dis brakes. The problem from what I have seen is that the original proportioning valve in my car (1967 disc setup) will not provide enought pressure to the new rear disc brakes since it is designed for rear drum brakes. I still do not see how adding a valve into the rear line will give more pressure. I would like it to be that simple.
Joined: Wed Aug 11 2010, 10:15AM
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Posts: 1230
flj wrote ...
Mike66Chryslers wrote ... flj, Even though the front brake circuit goes through the factory 1973 prop valve, the prop valve only affects the rear brake circuit, just as the aftermarket one does. There are two reasons that the factory prop valve has the front brake lines going into it:
1. The government mandated a warning light on the dashboard to indicate if one of the brake circuits failed. There is a slider block inside the unit that will slide to one side and close an electrical contact if one circuit loses pressure. You do not care about this. This slider block does not do anything magical like increase brake pressure to the circuit that has not failed. It only closes an electrical switch.
2. The factory prop valve replaced having a separate tee for the front brake lines as a production expedient. You can safely use a brass tee for the front brakes. No problem.
DO NOT plumb two prop valves in series. You are asking for trouble.
EDIT: I just noticed that you are converting a 67 Polara, which will already have dual-circuit brakes and a distribution block with the slider inside to activate an idiot light on the dash. (On a drum/drum car, this is a simple distribution block, not a prop valve.) Retain this block, connect the disc master cylinder in place of your existing drum MC, and plumb the adjustable prop valve into the line going to the rear brakes AFTER the distribution block. Done!
Next question: What brake booster are you going to use? Your original drum brake booster will not suffice.
The car is a 1967 Polara Convertible which had the Budd front discs. I will be replacing the Budd discs with the 1973 Kelsey Hayes system. My booster should be adequate for the dis brakes. The problem from what I have seen is that the original proportioning valve in my car (1967 disc setup) will not provide enought pressure to the new rear disc brakes since it is designed for rear drum brakes. I still do not see how adding a valve into the rear line will give more pressure. I would like it to be that simple.
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Posts: 2919
Yes, your current booster is fine.
I don't know enough about the specifics of the Budd system to tell you the easiest way to modify it, but definitely get rid of your original prop valve and install an aftermarket one (on the rear brake line) if switching to rear discs. Quote from the article that you linked to:
...be advised that they should NEVER be installed if the factory unit is still in place. Proportioning valves in series with one another can do nasty, unpredictable things!
Lets try a different approach to explain how it works: Don't think of absolute psi, but percentage. Say that the line pressure to the front brakes is 100%. The front brakes always get 100% of whatever pressure is applied by the MC. The prop valve (installed on the line to the rear brakes) reduces the pressure to the rear brakes by a certain percentage. With the factory prop valve, that percentage is factory-set and non-adjustable. With the adjustable prop valve, you control the percentage.
As I said before, the prop valve is plumbed into the rear brake circuit. It does not affect the front brakes at all. Quote from the article that you linked to:
...be advised that they should NEVER be installed in-line to the front brakes. The effect would be to make your vehicle rear-biased before you could say �terminal oversteer.� Front brake line pressure should always be left alone � only the rear pressures should be considered for proportioning.
When one says that the adjustable prop valve can "increase" the pressure to the rear brakes, it really means that it can be adjusted to "not decrease" the pressure by as much as the factory prop valve, which is what you're looking for with the rear disc conversion. Your rear line pressure still can't go over 100% of the front pressure.
The adjustable prop valve on the rear line gives a higher percentage pressure versus the front brake circuit (where the front brake circuit gets 100% of applied pressure) when you adjust the prop valve to not reduce the rear circuit pressure as much. This is accomplished by simply turning a knob on the adjustable prop valve.
It really is that simple. If you still don't understand, I advise you against installing rear discs. Just swap your Budds for the 73 front discs and keep your rear drum brakes.
<span class='smallblacktext'>[ Edited Thu May 12 2011, 08:15AM ]</span>
Joined: Wed Aug 11 2010, 10:15AM
Location: E WA
Posts: 1230
Mike66Chryslers wrote ... Yes, your current booster is fine.
I don't know enough about the specifics of the Budd system to tell you the easiest way to modify it, but definitely get rid of your original prop valve and install an aftermarket one (on the rear brake line) if switching to rear discs. Quote from the article that you linked to:
...be advised that they should NEVER be installed if the factory unit is still in place. Proportioning valves in series with one another can do nasty, unpredictable things!
Lets try a different approach to explain how it works: Don't think of absolute psi, but percentage. Say that the line pressure to the front brakes is 100%. The front brakes always get 100% of whatever pressure is applied by the MC. The prop valve (installed on the line to the rear brakes) reduces the pressure to the rear brakes by a certain percentage. With the factory prop valve, that percentage is factory-set and non-adjustable. With the adjustable prop valve, you control the percentage.
As I said before, the prop valve is plumbed into the rear brake circuit. It does not affect the front brakes at all. Quote from the article that you linked to:
...be advised that they should NEVER be installed in-line to the front brakes. The effect would be to make your vehicle rear-biased before you could say �terminal oversteer.� Front brake line pressure should always be left alone � only the rear pressures should be considered for proportioning.
When one says that the adjustable prop valve can "increase" the pressure to the rear brakes, it really means that it can be adjusted to "not decrease" the pressure by as much as the factory prop valve, which is what you're looking for with the rear disc conversion. Your rear line pressure still can't go over 100% of the front pressure.
The adjustable prop valve on the rear line gives a higher percentage pressure versus the front brake circuit (where the front brake circuit gets 100% of applied pressure) when you adjust the prop valve to not reduce the rear circuit pressure as much. This is accomplished by simply turning a knob on the adjustable prop valve.
It really is that simple. If you still don't understand, I advise you against installing rear discs. Just swap your Budds for the 73 front discs and keep your rear drum brakes.
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The 1973 Kelsey Hayes brake calipers had a 2.75" piston. That gives it 5.939" of surface area on the piston. The Budd caliper has 4 pistons at 2.36" in diameter. That gives it 8.748" of surface area (you only calculate one side of a multi-piston caliper).
I'm not sure how big the master cylinder is on the Budd system but there's a good chance it will be too big for a Kelsey Hays setup. Pedal effort will be much higher even with power assist. You may want to look into the matching master cylinder and replace the factory proportioning valve with the adjustable unit.